The present invention relates in general to a direct-conversion, or zero-intermediate-frequency, receiver, and in particular to measuring and canceling DC offsets in a direct-conversion receiver.
Radio-frequency (RF) signals generally consist of a carrier wave having a carrier frequency modulated by a data signal having a signal frequency distribution. RF receivers are designed to receive the RF signal and extract the data signal for further processing. In standard heterodyne receivers, the data signal is extracted by mixing the received signal with the output of a first local oscillator operating at a frequency less than the carrier frequency, thereby generating an intermediate-frequency (IF) signal. The IF signal is then filtered and amplified before being converted to the baseband. Conversion to the baseband generally involves mixing the IF signal with the output of a second local oscillator operating at the intermediate frequency.
Recently, there has been increased interest in direct-conversion, or zero-IF, receivers as an alternative to heterodyne architectures. In zero-IF receivers, there is one local oscillator operating at the carrier frequency, and the received signal is converted directly to the baseband without IF signal processing. Such receivers typically require simpler analog components than heterodyne receivers (e.g., analog filters and amplifiers for zero-IF receivers operate in the baseband rather than at a nonzero intermediate frequency) and consume less power. Because zero-IF receivers can operate at lower power and be more easily integrated into monolithic systems than heterodyne receivers, zero-IF receivers are recognized as potentially very useful for applications where low cost, low power consumption, and small size are important, such as various wireless mobile handheld devices.
Zero-IF receivers, however, are susceptible to noise from sources that are either far less significant or entirely absent in heterodyne receivers. One important noise source in zero-IF receivers is DC offset, a nonzero voltage that appears at the mixer output in the absence of a data signal. DC offset is caused, for instance, by current leakage from the local oscillator (which operates at the carrier frequency) to the mixer or other RF components, e.g., an RF amplifier. This leakage current can be propagated into the mixer, leading to a DC offset in the baseband signal. After a subsequent analog amplification stage, the DC offset can saturate downstream components, such as analog-to-digital converters, resulting in an increased receiver error rate. Thus, zero-IF receivers generally require DC offset cancellation.
One solution is to provide a current (or voltage) source downstream of the mixer and upstream of other (baseband) analog components. The current source is configured to provide a current (or DC voltage) that cancels the DC offset. In cases where the receiver has variable gain, the current source may be adjustable (e.g., voltage controlled), with the voltage determined by a lookup table responsive to the gain setting. Such lookup tables must be properly calibrated. Ideally, the calibration is updated from time to time, as conditions affecting the DC offset may change over time.
Another solution, which may be used in conjunction with a current or voltage source, is to provide AC coupling between the RF and baseband analog components, e.g., by placing a capacitor in the signal path between the mixer and the baseband components. The capacitor filters out constant DC offsets. In the case of a receiver having variable gain, however, an abrupt change in the gain may cause an abrupt change in the DC offset that is not immediately filtered by the capacitor. Thus, in the case of a variable gain receiver with AC coupling, it is generally useful to provide a current source to cancel the DC offset, as described above. In AC-coupled systems, however, calibration of the lookup table is difficult because a change in DC offset generally causes a time-dependent response in the baseband components that is not readily converted into a measurement of the DC offset.
A calibration method that is effective in the presence of a time-varying system response to an abrupt change in an input is therefore desirable.